This application is based on French Patent Application No. 00 15 887 filed Dec. 7, 2000, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. xc2xa7119.
1. Field of the Invention
The invention relates to wavelength division multiplex fiber optic transmission systems and more precisely to cross-connecting or switching wavelengths in them.
2. Description of the Prior Art
The context of the present invention is that of optical switches, or cross-connect units, having a multigranularity architecture. The granularity concept reflects the data transmission capacity of the optical network. To switch the total traffic at the level of a switch, this type of architecture therefore takes account of different data transmission capacity levels. For example, one portion of the total traffic may be switched at the fiber level, which corresponds to a high level of granularity, another portion may be switched at the band of wavelengths level, which corresponds to an intermediate level of granularity, and a final portion may be switched at the wavelength level, which corresponds to a low level of granularity. Supplementary levels of granularity can also be taken into account.
Transmission capacity in fiber optic systems is increasing all the time, because the number of channels transmitted by wavelength division multiplexing in each fiber is increasing and the number of fibers per cable is increasing. This gives rise to the problem of routing and cross-connecting channels in switching devices and, to be more specific, the problem of the complexity of switching devices liable to be required to route an increasing number of channels.
K. Harada et al, xe2x80x9cHierarchical optical path cross-connect systems for large scale WDM networksxe2x80x9d, proposes an optical cross-connection architecture corresponding to a hierarchical optical path structure with two layers. The paper proposes grouping adjacent wavelengths to form bands and switching the bands. This solution limits the number of wavelength converters used in each optical cross-connect unit.
FIG. 1 shows the principle of demultiplexing adjacent bands.
In the FIG. 1 example, the fiber consists of 12 channels or wavelengths xcex1 to xcex12. Each band consists of four adjacent channels. The fiber is therefore demultiplexed into three bands each of four adjacent channels by means of a fiber to band demultiplexer Demux Fxe2x86x92B. Each band is then demultiplexed into wavelengths by means of a band to wavelength demultiplexer Demuxxe2x80x2 Bxe2x86x92W. A band switch BXC is disposed between the band stage and the wavelength stage, immediately ahead of the band to wavelength demultiplexers Demuxxe2x80x2 Bxe2x86x92W.
In the FIG. 1 example, the signal from the fiber is therefore filtered to separate the adjacent bands.
Accordingly, the demultiplexer Demux Fxe2x86x92B used for fiber to band demultiplexing is an adjacent band demultiplexer. This kind of multiplexer uses filtering with a wide pass band and the transfer function FTb shown diagrammatically in FIG. 1. The filtering applied by the adjacent band demultiplexer Demux Fxe2x86x92B isolates all of the channels of the same band. A first band therefore consists of four adjacent wavelengths xcex1 to xcex4, a second band consists of adjacent wavelengths xcex5 to xcex8, and a third band consists of adjacent wavelengths xcex9 to xcex12.
For band to wavelength demultiplexing, each band, after routing in the band switch BXC, is demultiplexed into four channels via the band to wavelength demultiplexer Demuxxe2x80x2 Bxe2x86x92W. Each demultiplexer Demuxxe2x80x2 Bxe2x86x92W used is a 1-to-4 deinterleaving demultiplexer. This kind of deinterleaving demultiplexer uses periodic filtering whose transfer function FTcxe2x80x2 is shown diagrammatically in FIG. 1, based on Mach-Zehnder filters or on array waveguide gratings (AWG). The channel filtering applied by the deinterleaving demultiplexer Demuxxe2x80x2 Bxe2x86x92W is therefore periodic filtering to isolate one channel in the band.
Accordingly, to obtain the wavelengths, it is necessary to use deinterleaving demultiplexers, i.e. demultiplexers in which filtering is effected by periodic filters of the type described above.
In the case of multigranularity optical cross-connection architectures, because of the presence of switching stages such as the band switch BXC, it is not possible to know a priori the adjacent band that will be demultiplexed at the input of each band to wavelength demultiplexer. A deinterleaving demultiplexer, in which the filtering is periodic, takes account of all the adjacent bands. This kind of demultiplexer does not depend on the band at the input. Thus all wavelengths can be demultiplexed.
Another way to cross-connect wavelengths in wavelength division multiplex fiber optic transmission systems is to define interleaved bands rather than adjacent bands. The French patent document whose title in translation is xe2x80x9cAN INTERLEAVED BAND OPTICAL CROSS-CONNECTION SYSTEMxe2x80x9d therefore proposes, for cross-connecting optical transmission channels, grouping the various channels or the various wavelengths into interleaved bands. In this case, the bands are formed of wavelengths or channels that are not adjacent.
FIG. 2 shows the principle of demultiplexing when interleaved bands are used. The fiber consists of 12 wavelengths xcex1 to xcex12. Three bands each of four channels are obtained by means of a fiber to band demultiplexer Demuxxe2x80x2 Fxe2x86x92B.
Thus the fiber is demultiplexed into three bands which are interleaved, i.e. one channel of one band is adjacent channels of other bands. Accordingly, a first band consists of the wavelengths xcex1, xcex4, xcex7 and xcex10, a second band consists of the wavelengths xcex2, xcex5, xcex8 and xcex11, and, finally, a third band consists of the wavelengths xcex3, xcex6, xcex9 and xcex12. The channels of the same band are separated by a constant spectral gap.
Each band is then demultiplexed into wavelengths by means of a band to wavelength demultiplexer Demux BOW. Before being demultiplexed, the bands are switched in a band switch BXC.
For the fiber to band demultiplexing, the fiber is demultiplexed into three interleaved bands by the fiber to band demultiplexer Demuxxe2x80x2 Fxe2x86x92B. The demultiplexer Demuxxe2x80x2 Fxe2x86x92B used is a 1-to-3 deinterleaving demultiplexer. This kind of demultiplexer uses periodic filtering, based on Mach-Zehnder filters or array waveguide gratings, whose transfer function FTbxe2x80x2 is shown diagrammatically in FIG. 2 and which isolates all the channels of the same band.
For band to wavelength demultiplexing, each band of wavelengths is demultiplexed into four channels by the band to wavelength demultiplexer Demux Bxe2x86x92W. Each demultiplexer Demux Bxe2x86x92W used is an adjacent band demultiplexer, i.e. a demultiplexer that uses filtering with a wide pass band. The transfer function FTc of this kind of channel filter is shown diagrammatically in FIG. 2.
To obtain the wavelengths, it is necessary to use adjacent band demultiplexers Demux Bxe2x86x92W because, as explained above, due to the presence of the band switch stage BXC, it is not possible to know a priori the interleaved band that will be demultiplexed. This is why all the wavelengths are demultiplexed using adjacent band demultiplexers with a wide pass band, regardless of the interleaved band at the input.
However, the teaching of each of the above prior art documents is unsatisfactory. The two prior art systems, one with adjacent bands and the other with interleaved bands, in fact require the same demultiplexing devices.
In particular, where the system with adjacent bands is concerned, to go from fibers to wavelengths it is first necessary to use an adjacent band demultiplexer Demux Fxe2x86x92B to obtain the adjacent bands and then deinterleaving demultiplexers Demuxxe2x80x2 Bxe2x86x92W to obtain the wavelengths. Where the system with interleaved bands is concerned, to go from fibers to wavelengths, it is first necessary to use a deinterleaving demultiplexer Demuxxe2x80x2 Fxe2x86x92B to obtain the interleaved bands and then an adjacent band demultiplexer Demux Bxe2x86x92W to obtain the wavelengths.
Using an adjacent band demultiplexer in which filtering is effected by a filter with a wide pass band leads to major problems in terms of filtering quality. In particular, it is difficult to obtain a filter of this kind that does not lose wavelengths between two successive bands.
The problem to be addressed is that of the ratio between the pass band and the rejection band of the filter. The pass band corresponds to the whole of the spectrum that is passed without being attenuated by more than a predefined number of decibels (dB), for example 0.5 dB or 3 dB. The rejection band corresponds to a spectral gap outside which the signal is attenuated by at least a predefined number of decibels (dB), typically 20 dB or 25 dB. The wavelengths that are between the pass band and the rejection band cannot be used as they are too attenuated to be used on the channel concerned of the band demultiplexer and not sufficiently attenuated to be used on other channels of the band demultiplexer. They are therefore xe2x80x9clostxe2x80x9d wavelengths.
The object is to obtain a ratio close to one, to lose the fewest wavelengths. To obtain a pass band to rejection band ratio close to one, the filter rising and falling edges must be steep, i.e. the shape of the filter must be perfectly rectangular. This kind of filter is technologically very difficult to make.
FIG. 3 illustrates this problem relating to the use of adjacent band demultiplexers and shows fiber to adjacent band demultiplexing. An adjacent band demultiplexer is therefore used, with filtering with a wide pass band.
The top part A of FIG. 3 shows an ideal filter with a perfectly rectangular shape. Part B shows the real shape of the filter. The rectangular filter shape is highly imperfect. Consequently, wavelengths at the edges of the band, normally taken into account in ideal filtering, are lost and cannot be used. The lost wavelengths are crossed through in FIG. 3.
Thus using a filter with a wide pass band leads to loss of wavelengths.
Also, the problem that the invention intends to solve is that of efficiently demultiplexing n different optical granularities, where n is at least equal to 3, avoiding the drawbacks of the prior art explained above, i.e. without losing wavelengths.
To this end, the invention proposes to use interleaved bands and to use only deinterleaving demultiplexers, i.e. demultiplexers in which the filtering is periodic. In accordance with the invention, these demultiplexers are used both for fiber to band demultiplexing and for band to wavelength demultiplexing.
The system according to the invention also applies to wavelength to band multiplexing and band to fiber multiplexing. To this end interleaving multiplexers are used in the system.
The invention therefore provides an optical demultiplexing system for demultiplexing a multiplex which has at least three levels of granularity and includes m interleaved bands of wavelengths each of which includes p wavelengths, which system includes a 1-to-m deinterleaving demultiplexer for demultiplexing the multiplex into m bands of wavelengths and a 1-to-p deinterleaving demultiplexer for demultiplexing each of the m bands of wavelengths into p wavelengths, and in which system the numbers m and p are mutually prime.
The invention also provides an optical multiplexing system for obtaining a multiplex which has at least three levels of granularity and includes m interleaved bands of wavelengths each of which includes p wavelengths, which system includes m p-to-1 interleaving multiplexers, each for multiplexing p wavelengths into a band of wavelengths, and an m-to-1 interleaving multiplexer for multiplexing the m bands of wavelengths into a fiber, and in which system the numbers m and p are mutually prime.
Other features and advantages of the invention will become more clearly apparent on reading the following description of one particular embodiment, which description is given with reference to the accompanying drawings.